During the summer of 1965, the CRREL cloud physics group carried out a study of fog modification by propane and dry ice seeding on the Greenland Ice Cap, an area with one of the lowest concentrations of atmospheric pollution on earth. The nuclei of supercooled fog droplets before seeding were observed using electron microscopy and electron diffraction methods. The nuclei of ice crystals formed by dry ice and propane seeding were also observed using the same technique. The nuclei of the supercooled fog droplets and the ice crystals formed by seeding were found to be hygroscopic sea salt particles that were easily distinguished from the clay mineral nuclei of natural snow crystals. The sublimation temperature of carbon dioxide is -78.5C. The boiling point of liquid propane is -44.5C at 1 atm pressure. It is concluded that the ice nucleation of supercooled fog by dry ice and liquid propane seeding is due to thermal effects. (Author).